Investigations on the regulation of normal intestinal immunity have described a fundamental property of the intestinal immune system that distinguishes it from other peripheral and tissue responses. "Physiological inflammation" is a state of immune regulation in the intestine characterized by hypo-responsive T lymphocytes. This unique, but molecularly undefined property indicates that active tolerance is an essential component of mucosal immunity. We propose that regulated mucosal T cell hypo-responsiveness mediates intestinal immune tolerance, and that unregulated T cell activation initiates inflammatory bowel disease (IBD). In striking contrast to peripheral blood T cells (PBT), which are activated via the CD3 pathway, intestinal lamina propria T cell (LPT) responses are dominated by an alternate pathway that uses the CD2 receptor. We have developed an in vitro model in which LPT can toggle between a hypo-responsive (tolerant) state and a responsive (protective) state of activation. Using this model, we demonstrated that membrane proximal signaling within two minutes of activation through the CD3 receptor complex is markedly reduced in tolerant, but not protective LPT. We have also described that cholesterol-rich membrane micro-domains, called lipid rafts, are not only structural components of the plasma membrane, but also localize proteins for the initiation of intra-cellular signal transduction in PBT. With our expertise in characterizing LPT, isolating lipid rafts, and delineating signal transduction pathways in T cells, we are immediately poised to study mucosal T cell tolerance in the normal mucosa and aberrant T cell activation in the IBD mucosa at a molecular level. Our ability to identify and characterize heterogeneity among lipid raft constituents in LPT will be combined with molecular, biochemical, and immunological techniques to test the following central hypothesis: Lipid raft heterogeneity on intestinal T cells modulates their signaling, activation, and function, thereby contributing to both normal mucosal immune tolerance and chronic inflammation. Our aims are a, follows: (1) evaluate the structural mechanisms by which lipid raft heterogeneity modulates positive and negative regulation of the TCR signaling complex in tolerant LPT. (2) Investigate the topological coupling of the cytoskeleton to lipid rafts, via interactions through CD2 in LPT. (3) Explore the biochemical regulation of the second messenger Ras-MAPK pathway in lipid rafts from LPT. (4) Examine changes in lipid raft modulation of T cell activation from IBD mucosa. We believe this unique approach will generate important new information on intestinal immune tolerance and IBD, by examining the regulation of membrane proximal events in LPT signaling and how these events change in the IBD T cell